1. Field
The present invention relates to a light source for a liquid crystal display (LCD) device, and more particularly, to a backlight unit for the liquid crystal display (LCD) device.
2. Discussion of the Related Art
Flat panel display (FPD) devices that are small, lightweight, and have low power consumption have been a subject of recent research in the advent of the information age. Among many kinds of FPD devices, a liquid crystal display (LCD) device is widely used for notebook computers and desktop monitors because of its excellent characteristics of resolution, color display and display quality.
In general, a liquid crystal display (LCD) device includes two substrates spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. Each of the first and second substrates includes an electrode, whereby the electrodes of the first and second substrates face each other. When a voltage is applied to each of the electrodes, an electric field is induced between the electrodes. Accordingly, an alignment of liquid crystal molecules of the liquid crystal layer is changed by the varying intensity or direction of the induced electric field. Thus, the LCD device displays an image by varying the transmittance of light through the liquid crystal material layer based upon the arrangement of the liquid crystal molecules.
Since the LCD device does not emit light and controls the transmittance of light, an additional light source is necessary. Accordingly, the LCD device displays images by disposing a backlight at a backside of a liquid crystal (LC) panel, providing the LC panel with light from the backlight, and transmitting the light according to the arrangement of the liquid crystal molecules.
The backlight may be classified into a direct-type and an edge-type depending on position of a light source with respect to a display area. In the direct-type backlight, light sources, such as lamps, are disposed right under the display area, whereby light from the lamps is directly irradiated on substantially an entire surface of the LC panel. Thus, the direct-type backlight does not need a light guide plate. On the other hand, in the edge-type backlight, since one or more lamp is disposed at one or more edge of the display area, to transmit light from the lamp toward a front side of the backlight, the light guide plated is required.
In general, the edge-type backlight may be used for notebook computers or LCD monitors because the edge-type backlight has less stained brightness, thin thickness, and low power consumption. The direct-type backlight may be used for high brightness display devices. A related art edge-type backlight will be described hereinafter with reference to attached drawings.
FIG. 1 is a cross sectional view of an edge-type backlight unit for an LCD device according to the related art. As illustrated in FIG. 1, in an edge-type backlight unit 1, a lamp 20 and a lamp housing 22 are disposed at a side of a light guide plate 24. The lamp 20 serves as a linear light source, and the lamp housing 22 surrounds the lamp 20. The light guide plate 24 changes linear light from the lamps 20 at the side thereof into planar light. Dot patterns 25 are formed at a lower side of the light guide plate 24 to form uniform planar light by diffusing light. The dot patterns 25 are formed by a printing method.
A diffusing sheet 18, first and second prism sheets 14 and 16, and a passivation film 12 are sequentially arranged over the light guide plate 24. A reflecting plate 26 is disposed under the light guide plate 24 to prevent leakage of the light.
The lamp 20 is a light source that is supplied with high voltage to generate visible light. The lamp 20 is classified into two types: a cold cathode fluorescent lamp and a hot cathode fluorescent lamp. The lamp 20 irradiates the visible light by emitting electrons from a cathode due to a voltage supplied from the outside and the emitted electrons then collide with fluorescent substances. The lamp housing 22 reflects light emitted from the lamp 20 toward the light guide plate 24.
The light guide plate 24 provides the light emitted from the lamp 20 or the light reflected at the lamp housing 22 toward a front side of the backlight unit 1 by total reflection. At this time, the dot patterns 25 at the lower side of the light guide plate 24 effectively diffuse the totally reflected light.
The diffusing sheet 18 comprises a polyester film and spherical patterns formed on the polyester film. The polyester film includes polyethyleneterephthalate (PET) and the spherical patterns include acrylic resin. The diffusing sheet 18 diffuses light diffusively reflected from the light guide plate 24 and uniformly provides the light to substantially the entire surface of an LC panel (not shown) to thereby equalize the brightness of the LCD device.
Each of the first and second prism sheets 14 and 16 comprises a polyester film and prisms formed on the polyester film. The polyester film includes PET, and the prisms include acrylic resin. Each of the prisms has a triangular cross-section. The first and second prism sheets 14 and 16 condense light dispersed in various angles passing through the diffusing sheet 18 toward a normal direction with respect to the LC panel (not shown). The first and second prism sheets 14 and 16 are arranged such that prism peaks of the first prism sheets 14 are perpendicular to prism peaks of the second prism sheet 16.
The reflector 26 under the light guide plate 24 includes a polyester material. The reflector 26 reflects light emitted from the lower side of the light guide plate 24 toward the upper side of the light guide plate 24.
In the above backlight unit, the diffusing sheet 18, the first and second prism sheets 14 and 16, and the passivation film 12, which function differently, are disposed between the light guide plate 24 and the LC panel (not shown). However, the sheets cost a substantial amount to fabricate, which thereby increases the cost of the overall backlight unit.
Using a prism light guide plate, which includes prisms at a surface thereof, has been proposed and developed. The prism light guide plate decreases the number of sheets used in the backlight unit.
FIG. 2 is an upper perspective view of a prism light guide plate according to the related art. As shown in FIG. 2, prisms 51, which have cross sections of inverted triangular shapes, are formed at a lower surface of a light guide plate 50. The prisms 51 of FIG. 2 have sides that are either symmetrical or non-symmetrical. The prisms 51 at the lower surface of the light guide plate 50 condenses light toward a front side of the light guide plate 50 better than the dot patterns 25 of FIG. 1, which thereby increases the brightness of the LCD device.
FIG. 3 is a cross-sectional view of a backlight unit including the prism light guide plate of FIG. 2 according to the related art.
As shown in FIG. 3, in the backlight unit 40, a lamp 70 and a lamp housing 72 surrounding the lamp 70 are disposed at a side of the prism light guide plate 50. The prism light guide plate 50 has prisms 51 at a lower surface thereof. A prism sheet 60 including prisms (not shown) at an upper surface thereof is disposed over the prism light guide plate 50 such that prism peaks of the prism sheet 60 are perpendicular to prism peaks of the prism light guide plate 50. A passivation film 62 is disposed over the prism sheet 60.
In the backlight unit 40 including the prism light guide plate 50, the diffusing sheet 18 of FIG. 1 and one of the first and second prism sheets 14 and 16 of FIG. 1 are omitted. Additionally, the prism light guide plate 50 condenses light to a greater extent than the light guide plate 24 of FIG. 1 including the dot patterns 25 to thereby provide a high brightness backlight unit 40.
FIG. 4A is a cross-sectional view of a liquid crystal display device including the backlight unit of FIG. 3 according to the related art. FIG. 4B is an enlarged view of the region “A” of FIG. 4A. In FIGS. 4A and 4B, a bottom case and a top case are not illustrated.
As shown in FIGS. 4A and 4B, a lamp 70 is disposed at a side of a prism light guide plate 50, and a lamp housing 72 covers the lamp 70 with an opening. The opening of the lamp housing 72 corresponds to the side of the prism light guide plate 50. The prism light guide plate 50 changes the path of light emitted from the lamp 70 at the side thereof toward an upper side of the prism light guide plate 50. The prism light guide plate 50 has prisms 51 of inverted triangular shapes on substantially the entire lower surface thereof. A reflector 76 is disposed over the lower surface of the prism light guide plate 50. A prism sheet 60, a passivation film 62 and a liquid crystal panel 80 are sequentially disposed over an upper surface of the prism light guide plate 50.
A lower end of the lamp housing 72 partly overlaps the reflector 76 and an upper end of the lamp housing 72 contacts an end of the prism light guide plate 50. The prism sheet 60 and the passivation film 62 are spaced apart from the upper end of the lamp housing 72 and are in contact with the upper surface of the prism light guide plate 50.
In the liquid crystal display device including the backlight unit 40 having the prism light guide plate 50, light condensed by the prisms 51 of the prism light guide plate 50 is transmitted upward the prism light guide plate 50. However, light leakage occurs at an edge of the liquid crystal panel 80, more particularly, at a light-entering portion of the prism light guide plate 50 due to the collimation by the prisms 51 of the prism light guide plate 50. Since the light emitted from the lamp 70 is incident first on the light-entering portion, the light in the light-entering portion is stronger and brighter than light at an opposite end of the prism light guide plate 50. Therefore, if the light at the light-entering portion is condensed and transmitted into the edge of the liquid crystal panel 80, a large amount of light leakage may occur at the edge of the liquid crystal panel 80.